1. Field of the Invention
The present invention relates generally to a wave-front aberration compensation unit, wave-front aberration compensation device, and optical pickup, and more particularly to such a compatible-type wave-front aberration compensation unit, wave-front aberration compensation device, and optical pickup that can be used with a plurality of optical media with different disk thicknesses and recording densities such as digital video disks (DVDs) and compact disks (CDs).
2. Description of the Related Art
Since an optical pickup to reproduce DVD contents would involve high-density recording six times to eight times that of CDs of digital information, including moving-picture and computer data onto the DVDs having the same 12-cm diameter as the CDs, it is necessary to substantially reduce the spot diameter of the laser beam required to read out the pit information as compared to the optical pickup required to reproduce the CD contents.
Therefore, the optical pickup which can reproduce DVDs can also read out the data recorded on DVDs, each side of which is formatted for a high-recording density of about 5-G bytes, approximately eight times that of CDs, by reducing the beam spot diameter, specifically by decreasing the laser beam source wavelength from a CD-spec value of 780 nm down to 650 nm or 635 nm and also by increasing the objective lens's numerical aperture (NA) from a CD-spec value of 0.45 up to 0.6.
CDs, which generally employ the same recording format as DVDs, should preferably be reproduced on a DVD player.
However, if the laser beam wavelength, lambda, is shortened and also the NA of the objective lens is increased for playing on a DVD player, even a slight tilt of the disk may give rise to a wave-front aberration, mainly comatic aberration, thus reducing the margin for the perpendicular displacement angle of the disk surface from the optical axis of the optical pickup, i.e., tilt angle.
Also, if the CD has a larger thickness than a DVD, for example, in such a case where a two-sheet DVD is 0.6 mm thick each and a CD is 1.2 mm thick and when an optical pickup (of the optical system) originally for reproducing DVDs is used to reproduce CDs, a wave-front aberration, mainly spherical one, occurs, substantially increasing the laser beam spot diameter.
Therefore, the conventional optical pickups (of the optical system) for DVDs have a problem that they cannot read out the data recorded on CDs.
To solve this problem, the following methods have been proposed: providing and switching two different objective lenses of one for DVDs and the other for CDs; inserting a compensation lens at the collimator to compensate aberrations due to disks; or by employing a two-focus lens utilizing a hologram as the objective lens.
However, the method of using two objective lenses or a compensation lens would complicate the mechanism and require a large installation space and, therefore, is not suitable for the miniaturization trend.
Also, the method of using a hologram must utilize diffraction and/or interference and also the multi-beam system, so that it has a low optical utilization efficiency and is easily influenced by interference because of the multi-beam system.
Moreover, even if either of the abovementioned methods is used to make up the system so as to be used on both DVDs and CDs, it is difficult to simultaneously carry out tilt compensation, which compensates the aberration due to disk tilt, so that a tilt compensation device must also be equipped.
If, however, a tilt compensation device is provided independently, another problem would be produced that the system would become large and expensive.